Heating system for electron beam furnace

ABSTRACT

An electron beam furnace heating system is described wherein an electron beam produced by an electron gun is directed to a target by a transverse magnetic field in the path of the beam. The field is established by a pair of bar-shaped pole pieces having their longitudinal axes in a common plane and the field of sufficient strength that the beam is deflected to emerge from the same side of the magnetic field as the side from which it enters.

219- 121 SR MTROQ; no 396902 united States Patent A [151 3,655,902Firestone eta]. I [4s] Apr. 11, 1972 [s41 HEATING SYSTEM FOR ELECTRON3,475,542 10/1969 Hanks ..13/31 BEAM FURNACE 2,777,958 1/1957 Le Poole..250/41.93 ux [72] Inventors: Alexander ii. Firestone, El Sobrante; P EB d Robert w. Fisk, Sunnyvale; Kurt 0. Ken- A'c'lheany nedy Berkeley, aof calm Assistant Exammer--R. N. Envall, Jr.

a Attorney-Fitch,Even,Tabin&Luedeka [73] Assignee: Air ReductionCompany, Incorporated,

. New 57 ABSTRACT [22 1 med: 1970 An electron beam furnace heatingsystem is described wherein [2 1] Appl. No.: 81,720 an electron beamproduced by an electron gun is directed to a I target by a transversemagnetic field in the path of the beam. [52] s CL 13/31 219/121 EB Thefield is established by a pair of bar-shaped pole pieces [51,1,130...::::::::::::::'"1:33:11.13311113331131: .....H05b7/00havingthmongiwdimlm acommon plane and the field [58 1 Field oiSearch..13/31; 219/121'EB; 250/495 D, of sufficient Strength that the beam isdeflected to emerge 1 5 93 ME from the same side of the magnetic fieldas the side from I which it enters. 56 R f c C'ted es I 9 Claims, 4Drawing Figures UNITED STATES PATENTS 3,270,233 8/1966 Dietrich ..13/3lX VIIIIIIl/IIIII. v )11/1111#111111111111/1111/ "IIIIIIIIIIIIIIIT'IIAPatented April 11, 1972 FIGZ I NVCNTOES ATTYS.

s .5 1 flnmwfl m N70 M mmr w M A k! FIG.4

HEATING SYSTEM FOR ELECTRON BEAM FURNACE This invention relatesgenerally to electron beam furnaces and, more particularly, to animproved heating system in such afurnace.

Electron beam furnaces of a variety of designs are useful in theprocessing of many metals, alloys or other materials, for example wherehigh standards of purity are to be achieved by outgassing or by avoidingreaction with oxygen and nitrogen, or where a substrate is to be coatedby vaporization and condensation of the material. Electron beams areaparticularly useful form of heating in that it is possible to injectheat into a melt locally. Electron beam furnaces typically include anevacuated enclosure, a heating system comprising one or more electronbeam guns with associated deflection means for directing and focusingthe beams, and a container for the molten material being processed.

Depending upon the particular type of processing being carried out, thecontainer for the molten material may take a variety of forms. In asituation where it is desired to evaporate the material in the containerand subsequently condense the material on a suitably supported substrateto coat the substrate, a typical container consists of an open toppedupright crucible. Electron beam heating enables the crucible itself tobe cooled and thereby form a skull of solidified molten material betweenthe crucible and the molten material. This protects the purity of themolten material and makes it unnecessary to use high temperaturerefractories for the crucible constructron.

Another type of processing is the purification of metals and alloysbypassing the molten material or alloy over a shallow hearth; Exposureto the vacuum with coincident electron beam heating of the surfacecauses many volatile impurities and occluded gases to be drawn off ofthe molten material and thereby produces a greatly purified product.

Other forms of containers which may be utilized in metal processinginclude tundishes, launders, and ladles for transferring molten materialbetween various points. Electron beam heating may be utilized tomaintain the material in a molten condition while in such containers.

During the processing of molten material in an electron beam furnace,vaporized material may present ionization problems or may coat thevarious parts of the electron beam gun, impairing its operation.Moreover, spalling of condensed materials from cool surfaces of thevacuum enclosure, and splashing and splattering of the molten materialfrom the crucible, may also impair operation of the electron beam gun.By positioning the electron beam gun underneath the container of themolten material and by utilizing transverse magnetic fields fordeflecting the electron beam through a curving path of l80 or more,contamination and shorting of the electron beam gun is minimized.

The heating of large surface areas in electron beam furnaces may beaccomplished by employing a large number of electron beam guns withseparate deflecting fields for each of the beams. If the furnaceenclosure is relatively large, this may present no significant problem.However, if space is at a premium, the positioning of means forestablishing transverse magnetic fields closely adjacent each other maybe extremely difficult due to mutual interference and distortion of thefields. j

During the processing of molten material in an electron beam furnace,'aswell as during the vaporization thereof, a considerable amount ofvaporous material may be generated. The amount of such material istypically at its maximum in the region directly above the molten target.Accordingly, it is undesirable to position means for establishingtransverse magnetic fields for deflecting the beam directly above themolten target. This is because the pole pieces or other means utilizedto establish the fields are susceptible of becoming heavily coated withcondensed vaporous material with the possibility of a consequent adverseeffect on the configuration of the magneticfield. Moreover, flaking offof such condensate may result in recontamination of the melt.

When pole pieces or other means for producing a transverse magneticfield for deflecting an electron beam are positioned off to one side ofthe region above the molten target, the condensation problem mentionedabove is lessened. Nevertheless, several factors limit the distance fromthe target material at which the deflecting means can be positioned.With too low an angle of incidence of the electron beam onto the target,surface structures adjacent the sides of the target surface, such asmagnetic shields or structural members, may block the path of the beam.in addition, with a very low angle of incidence, the impact area on thesurface of the target is spread out, resulting in a more inefficienttransfer of heat.

Accordingly, it is an object of this invention to provide an improvedelectron beam furnace heating system.

Another object of the invention is to provide an improved heating systemfor use in an electron beam furnace in which efficient use is made ofspace and power.

Another object of the invention is to provide an electron beam furnaceheating system of minimal complexity in which a plurality of electronbeams may be directed and controlled through a curving path to a target.

It is another object of the invention to provide an electron beamfurnace heating system which is particularly useful for heating largeareas of molten material.

Other objects of the invention will become apparent to those skilled inthe art from the following description, taken in connection with theaccompanying drawings wherein:

FIG. 1 is a full section side view schematically illustrating a heatingsystem constructed in accordance with the invention;

FIG. 2 is a view taken along the line 22 of FIG. 1; and

FIGS. 3 and 4 are plan views of alternate configurations for pole piecesin the heating system of FIGS. 1 and 2.

Very generally, the electron beam furnace heating system of theinvention comprises an electron beam gun 11 for producing an electronbeam. Means 12 are provided for directing the beam to a target 13. Thedirecting means include a pair of barshaped pole pieces 14 and 15 havingtheir longitudinal axesin a common plane and positioned to produce atransverse magnetic field in the path of the beam. Means 16 are providedfor energizing the pole pieces to produce a magnetic field of sufficientstrength that the electron beam is deflected to emerge from the sameside of the magnetic field as the side from which it enters. The fieldis also of sufficient strength to prevent the electron beam frompenetrating past a plane extending through the longitudinal axes of thepole pieces.

Referring now to FIG. 1, oneembodiment of the invention is illustrated.The electron beam furnace includes an evacuated enclosure, part of whichis shown at 20. The molten material target 13 is contained in anelongated container 17 which is cooled by circulating coolant inpassages 18 to form a layer or skull 19 of solidified material betweenthe molten material and the container walls. The container 17 isillustrated as a hearth into which molten metal flows from a launder 21(FIG. 2) at one end. The contents of the hearth are discharged at theother end through an opening 22 (FIG. 2), and the level of moltenmaterial in the hearth may be controlled by a weir, not shown. Othermeans for placing molten material in the hearth and removing ittherefrom may include such things as tundishes, siphons, or ladles.Between entry and exit, the'material flows slowly along the hearth andthereby has a very high exposure rate to the vacuum environment in whichthe illustrated apparatus is disposed.

The hearth type of arrangement illustrated provides a large surface areaof molten material with shallow depth for long times of exposure of themolten material to the vacuum. Such an arrangement is particularlyuseful in the purification of many types of steel and nickel base alloysas well as most of the refractory metals, such as columbium, tantalum,titanium, zirconium and others. Experiments have shown that manypurification reactions that involve differential vaporization phenomenaor other types of outgassing require residence times of many tens ofseconds with the molten surface exposed to very low pressures. In suchinstances the illustrated configuration, that of a long linear hearthwith the molten material flowing slowly along it, is particularlyadvantageous. Electron beams are utilized to prevent solidification ofthe material on the hearth as the material flows along the hearth, andto provide localized regions of high heat to vaporize impurities andproduce thermal stirring.

In the illustrated apparatus, a plurality of electron beams are utilizedto provide heating of the target molten material 13 in the hearth 17.The electron beams are produced by a plurality of similar electron beamguns, only one of which (11) is shown, distributed along and underneaththe hearth 17 at spaced intervals.

The electron beam guns may be of any suitable type. A preferred form,however, is that shown and described in U.S. Pat. No. 3,514,656 assignedto the assignee of the present invention. The details of such a gun areshown in connection with the gun I! in FIG. 1. It is to be understoodthat the other guns may be of identical construction.

Referring now to FIG. 1, the electron beam gun 11 includes an elongatedemitter 23 for producing electrons. The emitter is preferably a tungstenwire and extends between the supporting membersv 24 and 25. Means, notillustrated, provide a direct current potential across the members 24and 25, resulting in a flow of direct current through the emitter 23.The current flow raises the temperature of the emitter causing it toproduce free electrons.

The freeelectrons produced by the emitter 23 are reflected on threesides by a shaping electrode 26. The electrode 26 is insulated from theemitter support members 24 and 25 by insulating strips 27 and 28,respectively. The shaping electrode 26 is formed with an elongatedrecess 29 through which the emitter 23 extends. When the shapingelectrode is maintained at the emitter potential, by suitable connectionnot illustrated, the electrons produced bythe emitter 23 tend to moveout of the open end of the recess 29 and away from the shaping electrode26. i

The electrons leaving the recess 29 in the shaping electrode 26 areaccelerated into a beam by an accelerating electrode 31 and pass throughan opening 32 therein. The accelerating electrode 31 consists of a platewith two right angle extensions 33 and 34 thereon which are attached tosuitable mounting means, not illustrated. The plate 31 is maintained ata potential which is substantially more positive than the potential ofthe emitter and-the backing electrode to produce an acceleration of theelectrons. The result is a ribbon beam, that is, an electron beam havingan elongated cross section which is ideally a narrowv rectangle butwhich approximates a narrow oval. The beam has a major axis plane whichextends through the emitter. v

The electrons in the beam leave the emitter 23 at an acute angle in themajor axis plane. The axis of the beam is indicated by the dash-dot line36, which represents the center of the ribbon beam. A non-normalorientation of the initial electron path with respect to the emitter 23is caused by the high intensity circumferential field produced by d-cheating current passing through the emitter. After leaving the anodeopening 32, the electron beam 36 is deflected about 90 through a curvedpath by means of a transverse magnetic field. The transverse magneticfield is established in the initial path of the beam between a pair ofelongated bar-shaped pole pieces 37, only one of which is illustrated.The pole pieces extend generally parallel with the emitter 23 and eachother and are positioned on either side of the 'beam 36 parallel withits major axis plane. A magnet 38 extends between the upper ends of thepole pieces 37, and a magnet 39 extends between the lower ends of thepole pieces 37. The two magnets are identically oriented with regard totheir polarities, and are electromagnets connected to a suitable controlcircuit and power supply, not shown. The polarities are established withfield lines running perpendicularly into the plane of the paper asillustrated in the drawing, thereby causing an upward deflection of theelectron beam 36 as illustrated. The effect of the field posite edges ofthe beam, as shown by the dotted lines 40, toward each other in theplane of the curving path due to a longer path length of electronstoward the lower edge of the beam in the magnetic field established bythe pole pieces 37. The details of the deflecting and focusing of thebeam are set out more fully in the aforementioned patent.

After leaving the electron beam gun l1 and after being deflectedupwardly through a change in direction of about the electron beam ispassed through an opening 4] in a vapor barrier 42. Although the vaporbarrier 42 may not always be necessary, in situations where a largequantity of vaporous material is produced, the vapor barrier 42 providesadditional protection for the electron beam .gun 11. In addition to thevapor barrier 42, a separate pumping system (not shown) may be providedin the lower region of the furnace interior to thereby maintain theelectron beam gun I] in a substantially vapor-free environment. Thisenhances the life of the emitter of the electron beam gun and improvesthe quality of the beam as it enters the upper part of the furnacechamber.

In order to deflect the beam from its generally vertically upward pathin the upper part of the furnace chamber back down onto the surface ofthe target 13, the pole pieces 14 and 15 are provided. Similar pairs ofpole pieces 44, 45 and 46, 47 are provided for deflecting the electronbeams produced by the other guns, as may be seen in FIG. 2. Aspreviously mentioned, the electron beam is deflected back down onto themolten material in the hearth at an angle which is'preferably betweenabout 30 and 60 from the vertical. An angle greater than 60 with respectto the vertical generally results in a beam spot which is too spread outand, in addition, the beam may be interfered with by structures alongthe sides of the hearth. An incident angle of less than 30 with respectto the vertical may necessitate the positioning of the pole piecestooclose to the region directly above the hearth, where the tendency forcondensate to collect on the pole pieces is undesirably high.

The pole pieces 14 and 15 are energized to provide a magnetic fieldwhich extends transversely of the electron beam above the opening 41through which the beam emerges and passes upwardly into the upper partof the electron beam furnace chamber. The field includes a portion ofsubstantially uniform strength in the region directly between the polepieces. The field also includes upper and lower fringe regions where thelines of flux flow outwardly away from the uniform region, and whichdiminish in strength with distance from the uniform region. For thepurposes of the present invention, the upper fringe region may beignored, as only the lower fringe region influences the electron beam orbeams.

The pole pieces are positioned at such an'angle that the magnetic fieldestablished thereby deflects the beam back downwardly and over towardthe target material 13. The magnetic field between the slantedbar-shaped pole pieces is made sufficiently strong by the energizingmeans 16 so that the beam does not penetrate an appreciable distanceinto the region directly between the pole pieces, but rather isdeflected through a change in direction of roughly entirely or nearlyentirely within the fringe region of the field and emerges from themagnetic field on the same side as the side from which it entered. Thus,the field established by the pole pieces functions in a manner similarto an optical mirror.

Referring to FIG. 2, the lines of force of the magnetic field may beseen extending between the pole pieces 14 and 15. As may also be seen,the deflection of the electrons in theelectron beam takes place almostentirely within the curving or fringe portion of the magnetic fieldestablished below the region directly between the pole pieces. Becausethe electrons towards the outside of the curving path of the beam spenda longer period of time within the magnetic field and pass throughstronger regions of the magnetic field than the electrons toward theinner edges of the curving beam, a focusing action in the plane of thecurving beam path may occur. Under some circumstances, the focusingaction may be so strong as to produce a sort of nodal point withsubsequent difon the electron beam also produces a convergence of theop- 75 fusion of the beam. By proper adjustment of the magnetics nearthe electron beam gun, a sufficiently compact beam cross section may beachieved that any tendency for the beam to diffuse in the above manneris tolerable.

Due to the outwardly bowing lines of flux in the lower fringe region ofthe field, the divergence of the beam in the direction transverse to theplane of its curving path as it leaves the magnetic field is increased.This means that the beam deflected by the field will be defocused in thelateral direction and have a bigger impact dimension in the flowdirection on the target surface that it would have if it had beenprojected the same distance without deflection. This is tolerable,however, if the beam cross section is kept compact by proper adjustmentof gun magnetics as mentioned above. Such a compact beam may be achievedby suitable placement and energization of the pole pieces 37 at the gun11.

The pole pieces 14 and 15 are supported on a pair of pole pieceextensions 51 and 52, respectively. The pole piece extensions are of aferromagnetic material and have their ends opposite the pole pieces 14and 15 in contact with the ends of the magnetic core 53 of an energizingcoil 54. The energizing coil is energized by suitable current from theenergizing means The position and the shape of the pole pieces can bevaried, depending upon the particular furnace geometry, to achieve thedesired beam spot size and deflection characteristics. Condensatebuild-up may be easily compensated for by slight adjustment in the fieldstrength when required. The actual distance of the pole pieces from thetarget and from the electron beam source is dictated by'consideration ofthe desired impact angle of the beam and by the geometric spacing of thetarget and the gun. The depth of the pole pieces does not have asubstantial affect on beam trajectory, since the beam does not penetratethrough the region between the pole pieces. The size of the facing areasof the pole pieces, however, determines the power needed to produce therequired field strength. The required field strength, of course, dependson the gun power as well as the amount of deflection desired. The angleof the poles with respect to the horizontal depends upon the anglebetween the incoming electron beam path and the desired outgoing beamangle. Preferably, a plane p including the longitudinal axes of thepoles is about perpendicular to a plane which bisects the angle formedby the incoming and outgoing beam paths and which is perpendicular to aplane therethrough. A shield 60 of ferromagnetic material prevents thefield adjacent the coil 54 and pole piece extensions 51 and 52 fromaffecting the beam.

In order to deflect the electron beam and thereby provide sweep of thebeam impact spot across the surface of the molten material 13, thedirecting means include a suitable set of pole pieces 61 for providingorthogonal deflection of the electron beam as it passes just above theopening 41. Although the particular construction of the deflection means61 is not shown and may take any suitable form, it is preferred that thedeflection means 61 comprise four solenoidal type electromagnetic coilsarranged with their axes intersecting and defining a rectangular planarregion. The coils have cores of low magnetic reluctance which are incontact at each end to the cores of the immediately adjacent coils.Variation in the energization of the coils produces a change in thedirection of the lines of force of the field established by the coilsand therefore a change in the amount of deflection of the beam.

Due to the outwardly bowing lines of flux in the magnetic field producedby the pole pieces 14 and 15, a magnification of the deflection of thebeam produced by the operation of the deflection means 61 occurs on anaxis perpendicular to the poles. Thus, a very wide range of sweep isavailable in the dimension along the hearth, as may be seen in FIG. 2.In FIG. 2, the electron beams of all the guns are indicated by dashedlines. The impact areas on the target surface are shown as small ovalsor circles at the ends of the beams. By appropriate spacing of theelectron beam guns and their deflecting fields, some overlap of thesweep may be achieved for a desirable redundancy in the operation of thesystem.

The depth at which the electron beam penetrates the region directlybetween the pole pieces (the region of field uniformity), is limited tothat depth which would avoid undue limitation of the amount of beamsweep which is possible. Moreover, excessive penetration may result inthe beam becoming to diflicult to control. Accordingly, the strength ofthe magnetic field is selected to avoid penetration of the beam past aplane extending between the longitudinal axes of the two pole pieces.

The configuration of the pole pieces 14 and 15 may be varied so thatportions thereof are slanted inwardly toward each other. This may beaccomplished either by using additional dipoles 63 as shown in FIG. 3,or by making the pole pieces themselves of appropriate configuration asshown in FIG. 4. By the use of this type of design, a more uniform fieldstrength may be achieved since there may be less of a tendency for fieldstrength to fall off toward the ends of the pole pieces because of theirincreasing proximity.

As may be seen in FIG. 2, a spurious field is established between theadjacent pole pieces of adjacent pairs. Thus, a spurious field isestablished between the pole pieces 15 and 44 and between the polepieces 45 and 46. Because deflection of the beam takes place almostcompletely in the fringe region of each main field, the fringes of thespurious fields thus established are out of the path of the electronbeams, even at wide sweep angles. Thus, even though spurious fields areestablished between adjacent pole pieces in adjacent pairs, they produceno detrimental affect on the operation of the system.

Another advantage of the present system is that, if additional power isneeded, a separate and redundant system may be placed on the oppositeside of the hearth, thereby doubling the power for heating. Moreover, ifdesired, two electron beam guns may utilize the same pair of pole piecesfor deflection, as is shown in connection with the pole pieces 46, 47 atthe far right-hand edge of FIG. 2.

It may therefore be seen that the invention provides an improved heatingsystem in an electron beam furnace. The invention is particularly usefulin connection with a plurality of electron beams where a relativelylarge surface area is to be heated and where the beams are to be sweptover the surface area. The invention enables the pole pieces forestablishing deflecting magnetic fields to be placed closely adjacenteach other without adverse affect on the operation of the system. Thesystem is simple and reliable of operation.

Various modifications of the invention will become apparent to thoseskilled in the art from the foregoing description and accompanyingdrawings. Such modifications are intended to fall within the scope ofthe appendant claims.

What is claimed is:

1. In an electron beam furnace, a heating system comprising, an electronbeam gun for producing an electron beam, and means for directing thebeam to a target, said directing means including a pair of bar-shapedpole pieces having their longitudinal axes in a common plane andpositioned to produce a transverse magnetic field in the path of thebeam, said magnetic field having a region between said pole pieces ofsubstantially straight lines of force and having a fringe region ofbowed lines of force toward the direction from which the electron beamenters said magnetic field, means for varying the angle at which theelectron beam is injected into the said magnetic field produced by saidpole pieces to thereby vary the direction of the emerging beam, andmeans for energizing said pole pieces to produce a magnetic field ofsufficient strength that the electron beam is deflected to emerge fromthe same side of said magnetic field as the side from which it enters,and is prevented from penetrating past a plane extending through thelongitudinal axes of said pole pieces, said pole pieces being of a widthsuch that substantially all of the electron beam is deflected within thefringe region of said magnetic field.

2. A system according to claim 1 wherein said energizing means areadapted to provide a field strength sufficient to cause a change indirection of the beam through an angle of between about 105 and about165.

3. A system according to claim 1 wherein said angle varying meansproduce a variation in more than one plane.

4. A system according to claim 1 wherein said pole pieces aresubstantially parallel with each other.

5. A system according to claim 1 wherein at least a portion of each ofsaid pole pieces is disposed to slant inwardly toward the correspondingportion of the other of said pole pieces such that the space betweensaid pole pieces narrows in the direction of the region directly abovethe target.

6. A system according to claim 1 wherein said energizing means includean electromagnetic coil having a low reluctance core, means forming alow reluctance magnetic flux path from the ends of said core to therespective pole pieces, and a magnetic shield disposed between said coiland the path of the electron beam.

7. A system according to claim 6 wherein said path forming means includea pair of upright extensions of low reluctance material forming a fluxpath from the ends of said core to ends of the respective pole pieces,and wherein said magnetic shield is disposed between at least a portionof said extensions and the path of the electron beam.

8. A system according to claim 1 including a further electron beam gunpositioned to project an electron beam into said magnetic field on thesame side thereof as the beam produced by said first named electron beamgun, said energizing means being adapted to produce a magnetic field ofsufficient strength that the electron beam produced by said furtherelectron beam gun is deflected to emerge from the same side of themagnetic field as the side from which it enters and is prevented frompenetrating past a plane extending through the longitudinal axes of saidpole pieces.

9. A system according to claim 1 wherein the plane common to thelongitudinal axes of said pole pieces is about perpendicular to a planewhich bisects the angle between the incoming and outgoing electron beampaths and which is perpendicular to a plane including such paths.

1. In an electron beam furnace, a heating system comprising, an electronbeam gun for producing an electron beam, and means for directing thebeam to a target, said directing means including a pair of bar-shapedpole pieces having their longitudinal axes in a common plane andpositioned to produce a transverse magnetic field in the path of thebeam, said magnetic field having a region between said pole pieces ofsubstantially straight lines of force and having a fringe region ofbowed lines of force toward the direction from which the electron beamenters said magnetic field, means for varying the angle at which theelectron beam is injected into the said magnetic field produced by saidpole pieces to thereby vary the direction of the emerging beam, andmeans for energizing said pole pieces to produce a magnetic field ofsufficient strength that the electron beam is deflected to emerge fromthe same side of said magnetic field as the side from which it enters,and is prevented from penetrating past a plane extending through thelongitudinal axes of said pole pieces, said pole pieces being of a widthsuch that substantially all of the electron beam is deflected within thefringe region of said magnetic field.
 2. A system according to claim 1wherein said energizing means are adapted to provide a field strengthsufficient to cause a change in direction of the beam through an angleof between about 105* and about 165*.
 3. A system according to claim 1wherein said angle varying means produce a variation in more than oneplane.
 4. A system according to claim 1 wherein said pole pieces aresubstantially parallel with each other.
 5. A system according to claim 1wherein at least a portion of each of said pole pieces is disposed toslant inwardly toward the corresponding portion of the other of saidpole pieces such that the space between said pole pieces narrows in thedirection of the region directly above the target.
 6. A system accordingto claim 1 wherein said energizing means include an electromagnetic coilhaving a low reluctance core, means forming a low reluctance magneticflux path from the ends of said core to the respective pole pieces, anda magnetic shield disposed between said coil and the path of theelectron beam.
 7. A system according to claim 6 wherein said pathforming means include a pair of upright extensions of low reluctancematerial forming a flux path from the ends of said core to ends of therespective pole pieces, and wherein said magnetic shield is disposedbetween at least a portion of said extensions and the path of theElectron beam.
 8. A system according to claim 1 including a furtherelectron beam gun positioned to project an electron beam into saidmagnetic field on the same side thereof as the beam produced by saidfirst named electron beam gun, said energizing means being adapted toproduce a magnetic field of sufficient strength that the electron beamproduced by said further electron beam gun is deflected to emerge fromthe same side of the magnetic field as the side from which it enters andis prevented from penetrating past a plane extending through thelongitudinal axes of said pole pieces.
 9. A system according to claim 1wherein the plane common to the longitudinal axes of said pole pieces isabout perpendicular to a plane which bisects the angle between theincoming and outgoing electron beam paths and which is perpendicular toa plane including such paths.